1. Field of the Invention
The present invention relates to a lithium ion conductive glass ceramics which is high in ionic conductivity, thermally and chemically stable and easy for the production.
2. Description of the Related Art
The progress of electronics in the recent years is conspicuous, and downsizing, weight reduction and high performance of electronic appliances are being rapidly advanced. Then, the development of a battery with high energy density and long life is eagerly desired as a power source for such an appliance. Above of all, expectation for a lithium ion battery is getting large every day.
Lithium ion conductive glass ceramics disclosed in JP-A-11-157872 and JP-A-2000-34134 are known as an electrolyte material of a lithium ion battery. The lithium ion conductive glass ceramics are obtained by depositing a crystalline in the interior of a glass by the heat treatment of a raw glass having a specified composition; and therefore, pores do not substantially exist in the interior so that the ionic conduction are not hindered by the pores. Accordingly, the lithium ion conductive glass ceramics have a characteristic feature that they are excellent in ionic conductivity as compared with lithium ion conductive oxide ceramics. However, the glass ceramics disclosed in JP-A-11-157872 involves a problem that its raw glass is low in thermal stability; and although it has high lithium ion conductivity, it was difficult to increase the thermal stability of the raw glass. When the thermal stability of the raw glass is low, there is caused a problem that in casting a molten glass in a die and performing molding, breakage is easily generated in the glass, and hence, the thermal control at the time of glass molding must be strictly carried out, leading to an increase of the production costs. When the thermal stability is low, there is also caused a problem that devitrification is easy to occur at the time of glass molding. The devitrification which is caused at the time of glass molding renders it difficult to uniformly deposit a desired crystalline in a sequent heat treatment (crystallization), resulting in making it unable to obtain high lithium ion conductivity. In order to generate neither breakage nor devitrification at the time of glass molding, it was necessary to strictly control a thermal condition at the time of molding a raw glass and also to strictly control an outflow velocity, an outflow condition and so on of a molten glass such that an impact is not given to the molten glass at the time of glass molding as far as possible.
Also, in JP-A-2000-34134, for the purpose of enhancing the thermal stability of a glass, it is successfully made to largely enhance (Tx−Tg) which is employed for evaluating the thermal stability of a glass by the addition of an M′2O3 component (wherein M′ is one or two or more members selected among In, Fe, Cr, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu). However, the thermal stability has not been achieved yet to extent that the glass can be mass-produced at low costs on an industrial scale. Also, raw materials having the foregoing M′ component are high in a distribution price on the market, and it is not preferred to obtain the foregoing thermal stability by such a component.